1. Field of the Invention
The present invention relates to power supply circuits that produce a desired output voltage from an input voltage, and to LCD driver ICs/circuits and liquid crystal display devices provided with such power supply circuits.
2. Description of Related Art
In recent years, as information display means of electronic devices, liquid crystal display devices provided with active-matrix liquid crystal display panels (hereinafter “LCD panels”) have come to be used increasingly widely for their improved visibility and responsivity.
Some examples of conventionally known active-matrix LCD panels are TFT (thin-film transistor) LCD panels and TFD (thin-film diode) LCD panels that employ thin-film transistors and thin-film diodes, respectively, as active elements for driving liquid crystal cells.
The number of terminals of each active element of the TFD LCD panels is one fewer than that of each active element of the TFT LCD panels. In addition, the TFD LCD panels have a simpler configuration, provide a higher pixel aperture ratio (and hence higher light use efficiency), and operate with less electric power consumption than the TFT LCD panels. For these reasons, as display means of electronic devices (such as cellular phone terminals) that require high brightness and low electric power consumption, the TFD LCD panels have been receiving much attention and have already been put to practical use.
However, the optimal drive voltage of the thin-film diode varies with the ambient temperature with a given gradient due to its element characteristics, and the temperature gradient thereof varies over a wide range (for example, over a range on the order of −40 mV/° C. to −110 mV/° C.) due to, for example, variations in characteristics of the LCD panels. Furthermore, the optimal drive voltage of the thin-film diode has a nonlinear characteristic that the temperature gradient thereof sharply increases when the ambient temperature falls below a predetermined temperature. Thus, to keep a uniform display contrast of the TFD LCD panel, an appropriate voltage needs to be constantly applied to the liquid crystal cells thereof. For this purpose, the actual drive voltage of the thin-film diode of each LCD panel needs to be compensated optimally according to temperature.
As an example of a conventional technology related to the present invention, JP-A-H11-231350 (hereinafter “Patent Document 1”) discloses and proposes a liquid crystal display device that drives a liquid crystal cell formed between first and second substrates supporting a liquid crystal by means of a pixel-switching nonlinear resistor element formed on the first substrate. This liquid crystal display device is provided with: a monitoring nonlinear resistor element formed on the first substrate at the same time as the pixel-switching nonlinear resistor element is formed thereon; and temperature compensation means that adds temperature compensation to a condition for driving the liquid crystal cell based on a current-voltage characteristic of the monitoring nonlinear resistor element, the current-voltage characteristic being obtained by energizing the monitoring nonlinear resistor element.
As another example of a conventional technology related to the present invention, JP-A-H06-314076 (hereinafter “Patent Document 2”) discloses and proposes a liquid crystal display device provided with a first control circuit that has a temperature detecting element for detecting the temperature of a liquid crystal element and sets a drive voltage of the liquid crystal element according to an output value of the temperature detecting element and a second control circuit that sets a drive voltage of the liquid crystal element in a low temperature region based on the output value of the temperature detecting element and a previously set value, wherein an output voltage can be switched either to the voltage set by the first control circuit or the voltage set by the second control circuit at a certain temperature in the low temperature region.
Certainly, by adopting the conventional technology disclosed in Patent Document 1, it is possible to maintain high display quality even when the current-voltage characteristic of the thin-film diodes varies with temperature. Alternatively, by adopting the conventional technology disclosed in Patent Document 2, it is possible to obtain the optimal display contrast by providing a drive voltage needed by the LCD panel even in the low temperature region.
However, since the conventional technology disclosed in Patent Document 1 is so configured as to detect the ambient temperature on the LCD panel side, an extra signal line is needed between the LCD panel and a control portion (an LCD driver IC) to transmit a monitoring result obtained on the LCD panel side to the control portion side. This makes it difficult to make the liquid crystal display device thinner and lighter, and hampers the cost reduction thereof.
On the other hand, the conventional technology disclosed in Patent Document 2 simply compensates for the nonlinear characteristic of the optimal drive voltage of the LCD panel, and thus gives no consideration to variations in the temperature gradient that become more pronounced when thin-film diodes are used as active elements.